Gas phase coating of boron nitride nanotubes with polymers

ABSTRACT

Boron nitride nanotube (BNNT)-polyimide (PI) and poly-xylene (PX) nano-composites, in the form of thin films, powder, and mats may be useful as layers in electronic circuits, windows, membranes, and coatings. The processes described chemical vapor deposition (CVD) processes for coating the BNNTs with polymeric material, specifically PI and PX. The processes rely on surface adsorption of polymeric material onto BNNTs as to modify their surface properties or create a uniform dispersion of polymer around nanotubes. The resulting functionalized BNNTs have numerous valuable applications.

This application is the U.S. national phase of International ApplicationNo. PCT/US2017/043140 filed Jul. 20, 2017 which designated the U.S. andclaims priority to U.S. Provisional Application Nos. 62/364,490, filedJul. 20, 2016, and 62/427,506 filed Nov. 29, 2016, the entire contentsof each of which are hereby incorporated by reference.

STATEMENT REGARDING GOVERNMENT SUPPORT

None.

FIELD

The present disclosure relates to forming functionalized BNNTs, and inparticular, vapor deposited polymer materials and inorganic nanotubes,and in particular, boron nitride nanotube and polyimide,poly-p-xylxylene,

BACKGROUND

Polymer materials incorporating boron nitride nanotubes (BNNTs) aredesirable for their improved properties, including as examples, highstrength, good electrical insulation, potentially low dielectricconstant, and good thermal conductivity. However, they typically haverelatively low BNNT content and when in polymer/BNNT composite films,the film thickness is typically greater than 50 μm. Low BNNT content andsuch relatively thick films limit the usefulness of the compositematerial, and consequently they have limited applications. Generally,the terms “thin film” and “thin wafer” refer to composites having a filmthickness of about 50 μm or less, and are dense and/or compacted. Meshfilms, on the other hand, are generally porous when deposited. Typicalpolyimide films are produced through codeposition of polyamic acid (PAA)that is composited with BNNT. The resulting material loses structuralintegrity at loadings above about 40 w % because of inhomogeneity ofpolymer distribution. Thus, improved film uniformity and homogeneity, aswell as enhanced control over film thickness, are desired.

As described herein, gas phase deposition of PAA precursors allows forsurface adsorption of gases and polycondensation of chains homogenously.Commercial processes involve solvation of diamine and dianhydridemonomers in polar aprotic solvents to form the intermediate product,PAA, in a condensation reaction, followed by deposition and animidization process to create composites. Significant challenges informing BNNT-polyimide composites in solution result from the highquality BNNTs making the precursor materials too viscous, due to thelong fibril characteristics of the nanotubes and inhomogeneity ofcomposite films caused by agglomeration of like constituents.

Parylene (poly-p-xylxylene) conformal coatings have been utilized in theelectronics industry as moisture barrier protection. It would thereforebe desirable to coat BNNT surfaces with parylene. Surface coating BNNTswith parylene has applications in structural and thermal composites aswell as highly porous membranes. The process typically involvesvaporization of di-p-xylene around 175° C. whereby it is fed through apyrolysis furnace (600-700° C.) and evolved into p-xylene monomer andfed into a deposition chamber. Poly-p-xylene condenses on surfaces asthe monomers react resulting in a conformal coating. The ability tocreate quality coatings onto tube surfaces creates unique nanocompositesthat are functional as membranes when coatings are preformed onbuckypapers. Furthermore, poly-p-xylene coated nanotubes have differentsolubility properties and interfacial faces with polymer matrices.

SUMMARY

This disclosure describes methods of forming boron nitride nanotube(BNNT)-polyamic acid (PAA), polyimide (PI) and poly-p-xylene (PX)composites and other thermoplastic and thermosetting composites, and inparticular, processes to form BNNT-PX, PI, and PAA nano-composites withhigh compositions of BNNTs. The methods described herein may producethin films ranging from about 100 nm to about 100 μm (and above, ifdesired), and are particularly suited for forming thin film coatings onBNNT surfaces. The resulting functionalized BNNTs have a wide range ofvaluable applications. These films are useful for, as examples only,layers in electronic circuits and x-ray windows, among other valuableuses. Generally, the present approach involves the chemical vapordeposition (CVD) of polymeric material on nanotubes, and in particularBNNTs. It should be appreciated by those of ordinary skill in the artthat variations in the disclosed embodiments are contemplated and may bemade without departing from the present approach. CVD processes may beused for coating the BNNT material, which may be, as examples, one ormore of a BNNT puff ball, BNNT powder, BNNT buckypaper, BNNT woven fibermat, BNNT fibers, BNNT porous scaffolding, or BNNT densified wafers,with the monomers. Some embodiments may employ one or more heating stepsto drive polymerization and imidization, resulting in the PI coatings onthe BNNTs. The thermal transition temperature of the pro PAA monomers toPAA in gas phase methods is 170° C. or at around this temperature.Further thermochemical transitions occur at approximately 270° C. in thecyclization of PAA chains in the imidization reaction. Crystallinity andchain length may be tuned through gradient heating between 1 and 100°C./min with thermal plateaus to optimize reactions. Other thermosettingpolymers have similar behavior.

Likewise, PX may be deposited in a system that has a chamber forvaporization of PX precursor. The precursor may be pyrolyzed intomonomer and the temperature and pressure adjusted to allow the monomerto condense as PX onto BNNT surfaces. The process may involvevaporization of di-p-xylene at around 175° C. and then feeding thematerial through a pyrolysis furnace (at about 600 to about 700° C.).The material may then be evolved into p-xylene monomer and fed into adeposition chamber. Poly-p-xylene condenses on surfaces as the monomersreact resulting in a conformal coating. The surfaces may include BNNTmaterials within in the chamber. The BNNT material may be, for example,one or more of a BNNT puff ball, BNNT powder, BNNT buckypaper, BNNTwoven fiber mat, BNNT fibers, BNNT porous scaffolding, or BNNT densifiedwafers. In some embodiments, the BNNT material may be supported by atemperature-regulated structure, such as a scaffolding.

By utilizing high quality BNNTs, i.e., BNNTs having few walls, fewdefects, length to diameters typically over 10,000 (high aspect ratio),diameters less than 10 nm, highly crystalline and catalyst free, BNNT-PIand BNNT-PX can be created that are useful as electrically insulating,thermally conductive layers in electronic circuits and as thin windowsfor x-ray, vacuum ultraviolet, porous membranes, etc. equipment.

It should be appreciated that BNNTs functionalized according to anembodiment of the present approach have numerous advantageous uses.BNNTs surface coated in PI, PAA, and PX can be suspended in anon-solvent, composited into a thermoplastics and thermosets, compositedinto an epoxy, polyurethane, polystyrene, polyisoprene matrix and formedinto parts, sheets, coatings, and adhesives. The present approachfurther allows for drastically more uniform and homogenous thin filmcoatings.

Embodiments of a process for synthesizing functionalized BNNTs aredisclosed. Generally, a BNNT material is positioned on a support in achamber. The support may be temperature regulated, such that the supporttemperature may be controlled independent of the chamber temperature.The chamber may be heated to evaporate monomers in the chamber, allowingfor a gas phase deposition of monomers onto the BNNT material. Thesupport may be cooled to drive condensation of monomers on the BNNTmaterial, to form a functionalized BNNT material. The cooling may beselectively set to condense a specific monomer, while other monomersremain in the gas phase. The BNNT material may initially take the formof at least one of a BNNT puff ball, a BNNT powder, a BNNT buckypaper, aBNNT woven fiber mat, or a BNNT porous scaffolding.

In some embodiments, the deposition chamber may be a Knudsen cellconfigured to control the evaporation of the first monomer and thesecond monomer through temperature and pressure regulation within thechamber. In some embodiments, the deposition chamber may be connected toa vaporization and pyrolyzing chamber to produce p-xylene monomer fromdi-p-xylene.

Some embodiments may feature two or more monomers. The monomers may bemonomers of polyimide. In some embodiments, a first monomer may be ananhydride, and a second monomer may be a diamine. The first and secondmonomers comprise monomers of poly(p-xylene). As another example, thefirst monomer and the second monomer may be selected to form a polyamicacid film on the BNNT material. As yet another example, the firstmonomer may be diamine, and the second monomer may be an anhydride gas.The first and second monomers may be introduced into the chambersimultaneously, or alternatively introduced alternatingly into thechamber. As an example, the first and second monomers are introducedalternatingly into the chamber, and an alternating cycle between thefirst and second monomers is less than about 100 Hertz. If desired,monomers may be introduced initially at the same time, and latermonomers may be introduced in an alternating fashion. The inverse islikewise contemplated. Depending on the desired outcome, the process maycontinue for about one hour. In some embodiments, the functionalizedBNNT material may be imidized to form a polyimide coated BNNTnano-composited material.

It should be appreciated that the selected monomers may be feed into thechamber at a desired rate. For example, the feed rate of p-xylene may becontrolled by the vaporization rate of di-p-xylene. With respect to thefunctionalized BNNT material, poly-p-xylene coated BNNTs may function assurface modified nanotubes. Also, polyamic acid and polyimide coatedBNNTs may function as surface modified nanotubes. The functionalizedBNNT material may be processed into a desired form factor. For example,the functionalized BNNT material may be compressed to form a non-wovenmat. As another example, functionalized BNNT material may be suspendedin a non-solvent. The non-solvent solution may have at least one of ametal, a ceramic, and a polymer matrix material. Additional processingmay include, but is not limited to, vacuum filtering the functionalizedBNNT material and casting the functionalized BNNT material to form aporous non-woven mats.

Depending on the desired end application, one or more nanoparticles maybe absorbed within the functionalized BNNT material. The nanoparticlemay include one or more of a medicine, a metal, a ceramic, and asemiconducting material. The nanoparticle can be activated byelectromagnetic radiation, including photons, or by nuclear radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a reaction chamber for deposition of PAA andpoly-p-xylene into a BNNT mat, according to an embodiment of the presentapproach.

FIG. 2 shows the general reaction of a diamine with a dianhydride toproduce a polyimide and base chemical structure of dianhydrides anddiamines with some common functionalities used to make functionalizedBNNT-PAA and BNNT-PI materials.

FIG. 3 shows the general reaction of di-para-xylene that degrades topara-xylene then condenses as poly-para-xylene in a deposition chamber.

DETAILED DESCRIPTION

BNNTs functionalized under the present approach have numerous valuableapplications. For example, applications that require electricalinsulation and thermal conductivity will benefit from highlycrystalline, thermally stable composites of boron nitride nanotubes(BNNT) and polyimide (PI) and poly-p-xylene (PX). Examples includeelectronic circuits having single diodes to billion-element electronicintegrated circuits, membranes, and low-energy x-rays windows. Otherapplications include silicon wafer bonding material, substrates forprinted circuit boards, and heat sync coatings for circuit boards andelectrical components. These are merely examples of the numerouspotential applications for BNNT surface coated through gas phaseprocesses to form nano-composite materials. The term “nano-composite”generally refers to a nanotube that is surface coated with a polymer,altering its diameter because of the surface adsorption of polymers.Properties of standalone nano-composites of BNNT and PI are prized fortheir enhanced characteristics of thermal conductivity due to thestabilizing effect of the BNNT. Likewise, PX is surface stabilized andless chemically active when coated onto a BNNT. PX and BNNTnanocomposites are an alternative to polyethylene membranes. Embodimentsof such functionalized BNNTs, and of processes for synthesizing them,are described below. It should be appreciated that departure from thespecifically disclosed embodiments may be made without departing fromthe present approach.

The BNNTs may go through a purification step prior being placed on asurface for polymer treatments (for any of methods described herein).Purification of BNNTs may include acid treatment to remove boron,amorphous boron nitride and hexagonal boron nitride particulates withcontrolled pH or spectroscopic in situ analytics. While these impuritiesalso have high dielectric performance, their thermal properties are notbeneficial for dissipative applications. Therefore, purification of theinitial BNNT material may involve the following methods. Acids, such asnitric acid or other oxoacid and superacid variants, may be used.Additionally, the acid(s) may be at an elevated temperature such as, forexample, 30° C. to 200° C., to increase the reaction rate on activeregions, specifically crystal edges of BNNTs and impurities. The acidtreatment may be followed by ample rinsing with, for example, deionizedwater, to neutralize the product and prevent further oxidative reactionsand to remove the oxidized constituents. BNNT purification may involvefurther steps, such as those described in U.S. Provisional ApplicationNo. 62/427,506, which is incorporated by reference in its entirety. Forexample, purification may involve an oxygen feedstock to evolve unwantedboron and boron nitride to oxygen saturated borates. Simultaneously ahydrogen feedstock evolves the borates to hydrogen borates that sublimeat the elevated process temperature.

Contemporary methods produce films on BNNTs with insufficient thermalconductivity, because of the low BNNT composition. The present approachprovides methods for synthesizing BNNT-based PI and PX compositematerials. As described herein, these multi-step processes may be usedto synthesize BNNT-PI and BNNT-PX composites, and overcome thelimitations of low density of BNNT in polymer matrices. The resultingfunctionalized BNNTs have numerous advantageous uses.

Generally, embodiments of the present approach involve an initial BNNTmaterial. The BNNT material comprises at least one of a BNNT puff ball,a BNNT powder, a BNNT buckypaper, a BNNT woven fiber mat, or a BNNTporous scaffolding. The BNNT material may be prepared by, for example,by deposition of a thin film onto a substrate capable of resistingsurface interactions with the walls of the BNNTs, freeze drying purifiedboron nitride nanotubes into powder form or porous scaffolding,pelleting boron nitride nanotubes through compression, or evaporativedeposition or vacuum filtration of a BNNT suspension into a buckypaper.These BNNT form factors maintain porosities appropriate for permeationof monomer gases throughout the BNNT material, and allow for homogenoussurface coating of the nanotubes. Other BNNT materials may be suitable,provided that they include adequate porosity. Processes for forming theBNNT form factor typically leave impurities such as organic residues onthe surface of BNNTs. Thermal treatment may be used to remove residualsolvents on BNNT surfaces. Time intervals and temperature(s) for thermaltreatment may vary depending on the embodiment, but generally depend onthe type solvent and its heat of vaporization. Due to substrateinteractions with the walls of the BNNTs, substrates for thin films ofBNNT may be selected for optimization of adhesion or exfoliationproperties depending on the application and successive methodfabrication techniques. Suggested substrates for deposition/filtrationand successful exfoliation include, for example, undoped silicate,aluminum, silicon, and n-doped silicon and aluminum oxide wafers andfilters. If the BNNT-PI and BNNT PX composite material is to be removedfrom the substrate, p-doped functioning and polymeric materials may notbe the most suitable for exfoliation. If the final BNNT-PI and BNNT-PXfilm is to remain on the substrate, the substrate material may beselected to optimize adhesion of the film to the substrate; for example,boron doped silicon will have higher adhesion than phosphorous dopedsilicon. In some embodiments, the substrates may have root mean square(RMS) roughness under 100 nm for roll-to-roll exfoliation and othertechniques that require low friction and mechanical hindrances. Ascanning tunneling (STM), atomic force microscopy (AFM), and surfaceprofilometer equipment are generally used to measure surface topography.An RMS roughness is the average variance of surface height across ascanned two-dimensional section where the measured variable is thez-axis corresponding to surface height. Selection of substrates forcalendaring of the BNNT-PI and BNNT-PX should have melting temperaturesabove 250° C.

BNNT material may be processed further prior to coating, which may beuseful for certain form factors. For example, following powderizationthrough freeze drying or deposition as buckypaper and removal of thesolvent, the resultant BNNT mat may be calendared to reduce itsthickness. Some BNNT synthesis processes produce a BNNT puffball formfactor, which may also be used. The calendaring surface may be n-dopedsilicon or similar material so that it is removable following thecalendaring step in the process. The process of calendaring involves thecompression of a deposited film to increase density and decreaseporosity. It is preferred to fabricate nanocomposites of polymer on BNNTon clean and purified nanotubes with higher surface area form factorssuch as the original puff ball because of the fibrilization that occurswhen the material is allow to agglomerate. Varying levels of compressioncan be achieved with hydraulic and mechanical presses. The band gap ofBNNTs is around 5.7 eV. Improved dielectric properties can be achievedthrough improvements to porosity, such as in as deposited films thatcontain a larger amount of void regions that are electrically isolating.However, more dense films of BNNTs will have higher thermalconductivity. The calendaring process may also result in some in-planealignment of the BNNTs in the plane of the substrate.

BNNTs in varying form factors may be composited with pro-PAA monomer,PAA, pro-poly-xylene monomer, or poly-xylene. Embodiments describedherein involve surface adsorption of the monomer to a BNNT material,which may then undergo further thermochemical processes to synthesizeBNNT-composites. In some embodiments, the methods may be used tosynthesize sulfonated variants of BNNT-PI or ortho, metaBNNT-poly-x-parylene composite materials. In some embodiments depositionis over a porous thin film of BNNTs followed by calendaring. In otherembodiments monomer deposition may be performed over a precalendaredBNNT thin film.

After BNNT material deposition, the BNNT material may undergo surfacetreatment with pro variant PAA or pro variant PX monomers in gas phase.Monomer deposition in the gas phase dramatically improves uniformity andhomogeneity of the film, and when regulated also allows for depositionof thin and ultra-thin films. These processes may be performed in, forexample, a Knudsen cell, or alternatively in a cell that allows gasmaterial to fill a chamber and condense onto BNNTs loaded onto asubstrate. Spassova describes a process for utilizing CVD to synthesizePAA and furthermore PI. See Spassova, E. “Vacuum deposited polyimidethin films”, Vacuum. 70, pp. 551-61, (2003). However, Spassova merelyperforms a CVD process for coating items such as a sheet of silica withPI. Spassova's process does not provide nanoscale conformal coatings,and would be inadequate for forming functionalized BNNTs as taught bythe present approach.

Under the present approach, the process permits uniform deposition ofdissimilar monomers on nanotubes, including form factors of BNNT puffball, a BNNT powder, a BNNT buckypaper, a BNNT woven fiber mat, or aBNNT porous scaffolding. FIG. 1 illustrates an embodiment of a modifiedKnudsen cell 11 containing a BNNT mat 12 applied to a substrate 13.Holder 16 supports BNNT mat 12 and substrate 13 within the cell 11.Typically, cell 11 is evacuated at start-up and maintained at partialpressure during the process. During CVD processing, PAA and PX monomerconstituents, to be discussed in FIGS. 2 and 3, such as ODA 14, PMDA 15,are heated to evaporate the monomers into the Knudsen cell 11 orpyrolyzing furnace (not shown). Monomers may be co-evaporated, or may bealternatingly evaporated at a desired rate. For example, someembodiments may involve an alternating cycle between the first andsecond monomers that is less than about 100 Hertz. It should beappreciated that the monomers may be varied without departing from thepresent approach. The temperature of the Knudsen cell 11 should besufficiently high to preclude monomer condensation or collection on thewalls of the Knudsen cell 11. The substrate 13 may be held at asufficiently low temperature to drive polycondensation of monomers 17and 18, collection on the substrate 13 and on the BNNT mat 12. Thesupport or holder 16 may be heated to maintain a temperature similar tothe Knudsen cell 11 temperature, while the upper surface of the holder16 may be slightly cooled to drive the monomers 17 and 18condensation/collection on the substrate 13 and the BNNT mat 12. Heatingand cooling loops 110 and 111 may be used to heat and cool the holder 16and the substrate 13 such that the monomers 17 and 18 collect only onsubstrate 13 and BNNT mat 12 surfaces. Alternate embodiments may usethermal electric elements to provide heating and cooling to the holder16 and substrate 13. An infrared radiant element 19 may be present tocreate a temperature gradient across the BNNT mat 12. A temperaturegradient may control the preferential collection of monomers 17 and 18,so that, for example, monomers 17 and 18 preferentially collect from thesubstrate 13 side of the BNNT mat 12, through the BNNT mat 12, and thenfinally on the external (e.g., top) side of the BNNT mat 12. In someembodiments, the substrate 13, BNNT mat 12, holder 16, and infraredheater (if present) may be inverted, such that the CVD process proceedsdownwards rather than upwards, would occur in the configuration shown inFIG. 1. In some embodiments the substrate 13, BNNT mat 12, holder 16 andassociated support (not shown), heating and cooling components may berotated or oscillated such as to assist in making the CVD processuniform across the entire surface area of the BNNT mat 12. It should beappreciated that the BNNT material (e.g., the form factor), may bedifferent than the mat 12 shown in FIG. 1, without departing from thepresent approach.

In some embodiments, the amounts of ODA 14 and PMDA 15 monomers used inthe process are of generally equal molar value or with minimally excessdianhydride: diamine (e.g., 52:48 w:w), and controlled to supply thedesired level of CVD to the BNNT mat. In some embodiments, including anadditional thin layer or layers of monomers, may also include additionalmaterial for forming a thin layer of monomers 17 and 18. In someembodiments, the additional thin layers of monomers 17 and 18 (which maybe the same monomers or may be new monomers, for example) deposit acrossthe outer layer of the BNNT mat 12. The additional material for theouter layer may be desirable so as to create a smooth, chemicallyhomogenous final surface. Relative monomer amounts may be adjusted togenerate the desired end product. In some embodiments, the additionallayers may of different chemistry to include molecules that may be ofmedical use, metalloids for creating metal groups or quantum dots on thesurface, molecules or atoms that may have catalytic properties, andmolecules or atoms that may be excited by electromagnetic radiation, toinclude photons, or nuclear radiation. In some embodiments, the monomersboth for the initial layers and the possible additional layers by beintroduced cyclically where the relative vapor pressures of the monomersare varied in time, the temperature of the walls of the cell are variedin time, and the temperature and temperature profile of the scaffoldholding the BNNT mat is varied in time. As one skilled in the art of CVDis aware, the times, temperatures and pressures of all of the componentsof the system all affect the CVD process.

Following the CVD process for collecting the PAA monomers 14 and 15 onthe BNNTs in the BNNT mat 12, and the outer coating (if present),calendaring may be used to decrease film thickness of the BNNT mat 12with the collected monomers 17 and 18. Calendaring of BNNT mats beforemonomer treatment and before PAA conversion of pro PAA monomers at 100°C. to 250° C. are considered. Next, thermal treatment may be used toform PAA intermediate and final PI throughout the BNNT mat 12 via thepolymerization and imidization processes. The important thermaltransitions (thermochemical reactions) of pro PAA monomers are 100° C.to 200° C. (polycondensation of dianhydride and diamine monomers) andbetween 220° C. and 300° C. for imidization of PAA. For example, in someembodiments, thermal treatment may be carried out in intervals between100 and 300° C. on the order of 1-100° C./min over the intervals andholding at the desired temperature for optimization that includesimprovement to PI molecular weight and crystal grain size. In someembodiments, the final BNNT-PI films may be exfoliated, though roll toroll processing, contact resist exfoliation, and embodiments thatinvolve removal by rinsing the film off of the deposition surfaces inall methods.

PX coatings on BNNTs may be synthesized in a similar manner. Clearly,the starting monomer is different. PX is typically deposited from adimer feedstock, for example ortho, meta, or para xylene, the arenesubstritutions. Di-para-xylene is the most common feedstock forpreparation of xylene monomer feedstock. PX monomers are prepared viavaporization of dimer at 40 to 200° C. into a pyrolyzying furnace thatis between 400 to 700° C. After pyrolyzing, the monomer exists as themonomer of the dimer used. The monomer condenses onto surfaces withinthe deposition chamber with the same arene functionalization as thefeedstock.

For BNNT-PI, BNNT-PAA, and BNNT-PX films that are to be removed from thesubstrate a resist may be utilized. Resists are defined as materialsdesired for thin film processing that are easily removed to obtaindesired films or other form factors. Resists in some embodiments may besolvated for easy removed through rinsing. These processes depictpolymer or metallic films that may be etched through solvent or acidsolvation. For example, an aluminum film may be used in the calendaringprocess and subsequently removed by, for example, phosphoric acid, thenrinsed away leaving behind a calendared BNNT wafer. In some embodiments,monomer heat treatment, treatment converting monomers to PAA, can beperformed after isolation and drying of the BNNT film onto a filtrationmembrane. The substrate used for monomer treatment and support for acidtreatment may vary depending on the embodiment, and may depend onwhether the membrane is to be part of the final BNNT-IP film, or insteadthe membrane is to be removed from the BNNT-PI film. In someembodiments, filtration membranes that remain in the BNNT-PI film may beformed from materials with melt temperatures above 200° C., and havesignificant acid stability. Otherwise, the filtration membrane maycontaminate the resulting BNNT-PI film. This process may significantlydecrease polyimide composition as compared to embodiments of describedabove. However, for purposes of binding BNNTs for successful exfoliationof films from the substrate, calendaring after monomer treatment may beused in some embodiments.

FIG. 2 shows the chemical processes for preparation of PAA and PI. Thereaction of diamine and dianhydride progress as shown in FIG. 2.1.Embodiments of the methods described herein may include variants ofdiamine and dianhydride monomers on the basis of varied R-groups, suchas the examples shown in FIGS. 2.2, 2.4 and 2.3, 2.5 respectively. Otherembodiments may utilize other R-groups. FIG. 2.1 shows the monomers ofPI and PI final chemical structure dehydration polymerization reaction.FIG. 3 shows the chemical processes for preparation of PX.

The techniques involving solutions of monomers into solvents and gasphase depositions of pro PAA and PX monomers may require pretreatment ofmonomers to reduce water concentrations in the reaction cell. Waterundesirably terminates propagation of PAA chains. Dehydration of themonomers may be performed prior to dispersion or monomer treatments toreduce detrimental termination.

In general, gas phase CVD produces longer chains of PX, PAA intermediateand PI final product, compared with wet chemistry processes. Gas phasedeposition is preferable over liquid phase depositions of PAA or pro PAAmonomers because anhydrous environments are preferred to synthesize highdensity and crystallinity in PI chains. Thermal treatment reduces theenergy of the final product, through increases in crystallinity that areoptimally chemically stable, and results in a highly crystalline BNNT-PIcomposite film. It should be appreciated that higher crystallinity inthe BNNT-PI composite material results in high thermal conductivity, andthus enhancing crystallinity leads to optimal thermal conductivitythrough improvements to grain sizes and phononic channels that increasesphonon mobility. Additionally, the BNNT sidewalls function as crystaltemplates for aiding propagation of the PAA resulting in higher PAAchain lengths along the BNNT's surfaces.

Other polymers may deposited and composited with the BNNTs via CVD in amanor similar to the monomers going into the PI. The temperature leveland temperature gradient described herein, where a temperaturedifference is created across the BNNT layer by cooling on one side ofthe mat and heating on the other side of the mat, can be used to controlthe rate of deposition across the BNNT layer and for a final surfacecoating of the polymers. Calendaring under vacuum or reduced pressuremay also be utilized to reduce voids.

Alignment of BNNTs in the substrate plane and out of plane is importantfor enhancements to thermal conductivity. Depending on the desiredthermal dissipation parameters, tube orientation will be manipulated tosufficiently act as phononic pathways. Orientation of BNNT mats istypically randomly orientated and may suffice for out of plane thermalconductivity and calendared BNNTs orient in plane for in plane thermalconductivity. Additionally, BNNTs are chemically inert support materialsthat may also function as a capsule. The hollow cavity within a BNNT canabsorb nanoparticles, such as, for example, medicines, metals, ceramics,and semiconducting nanoparticles, and protect such nanoparticles fromchemical degradation. BNNTs absorb solvent readily, therefore whennanoparticles are dispersed into solvents they are absorbed intonanotubes. Encapsulating the entirety of a BNNT with PX or PI allows fora packaging of species that may degrade and be constituted ofbiocompatible polymer or may be further functionalized to bebiocompatible.

The methods described in the present approach may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. The disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive by theforegoing description.

We claim:
 1. A process for synthesizing functionalized boron nitridenanotubes (BNNTs), the method comprising: positioning a solid BNNTmaterial on a support in a chamber, the solid BNNT material comprising aplurality of BNNTs having surfaces; heating the chamber to evaporate afirst monomer and a second monomer in the chamber; cooling the supportto drive condensation of the first monomer and the second monomer ontosurfaces of the BNNTs in the solid BNNT material to form a solidfunctionalized BNNT material comprising surface-coated BNNTs.
 2. Theprocess of claim 1, wherein the solid BNNT material comprises at leastone of a BNNT puff ball, a BNNT powder, a BNNT buckypaper, a BNNT wovenfiber mat, and a BNNT porous scaffolding.
 3. The process of claim 1,wherein the chamber comprises a Knudsen cell configured to control theevaporation of the first monomer and the second monomer throughtemperature and pressure regulation within the chamber.
 4. The processof claim 1, wherein the first monomer and the second monomer comprisemonomers of polyimide.
 5. The process claim 4, wherein the first monomercomprises an anhydride, and the second monomer comprises a diamine. 6.The process of claim 1, wherein the first monomer and the second monomerare selected to form a polyamic acid film on the BNNT material.
 7. Theprocess of claim 1, wherein the first monomer comprises diamine, and thesecond monomer comprises anhydride gas, and the first and secondmonomers are one of introduced into the chamber simultaneously, orintroduced alternatingly into the chamber.
 8. The process of claim 1,wherein the process continues for about one hour, the first and secondmonomers are introduced alternatingly into the chamber, and analternating cycle between the first and second monomers is less thanabout 100 Hertz.
 9. The process of claim 6, further comprising imidizingthe functionalized BNNT material to form a polyimide coated BNNTnano-composited material.
 10. The process of claim 9, wherein thefunctionalized BNNT material is imidized.
 11. The process of claim 1,wherein the first and second monomers comprise monomers ofpoly(p-xylene).
 12. The process of claim 1 wherein the depositionchamber is connected to a vaporization and pyrolyzing chamber to producep-xylene monomer from di-p-xylene.
 13. The process of claim 12, whereinthe feed rate of p-xylene is controlled by the vaporization rate ofdi-p-xylene.
 14. The process of claim 11, wherein the poly-p-xylenecoated BNNTs function as surface modified nanotubes.
 15. The process ofclaim 9, wherein the polyamic acid and polyimide coated BNNTs functionas surface modified nanotubes.
 16. The process of claim 1, wherein thefunctionalized BNNT material is compressed to form a non-woven mat. 17.The process of claim 1, further comprising suspending the functionalizedBNNT material in a non-solvent.
 18. The process of claim 17, wherein thenon-solvent solution comprises at least one of a metal, a ceramic, and apolymer matrix material.
 19. The process of claim 11, further comprisingat least one of vacuum filtering the functionalized BNNT material andcasting the functionalized BNNT material to form a porous non-wovenmats.
 20. The process of claim 1, further comprising absorbing ananoparticle within the functionalized BNNT material.
 21. The process ofclaim 20, wherein the nanoparticle comprises one or more of a medicine,a metal, a ceramic, and a semiconducting material.
 22. The process ofclaim 20, wherein the nanoparticle can be activated by electromagneticradiation, including photons, or by nuclear radiation.
 23. The processof claim 1, further comprising cooling the support to establish atemperature gradient across the solid BNNT material, thereby controllingthe condensation of the first monomer and the second monomer ontosurfaces of the BNNTs in the solid BNNT material.